This invention relates to a laser scanner in accordance with the preamble of claim 1.
Such a laser scanner or laser radar device is known from DE 43 40 756 A1. In that laser radar device, a pulsed laser directs controlled, successive light pulses into a monitored region. A light receiving arrangement detects light pulses that are reflected by an object in the monitored region and generates corresponding electric signals which are fed to an evaluation unit. The evaluation unit determines the distance between the object and the laser radar device and generates a representative distance signal on the basis of the time interval between the emission and receipt of the light pulse on the basis of the speed of light. A light diverter is arranged between the pulsed laser and the monitored region which directs the light pulses to the monitored region while continuously changing the direction of the pulses so that the entire monitored region is traversed by the light pulse. This is particularly useful for detecting objects within a dangerous zone. Due to their ability to determine distances, such laser radar devices detect not only the presence of an object; with the obtained distance information and the rotational angle of the light diverting unit, the precise position of the object can be determined.
The word “light” is not limited to visible light. For purposes of this application, “light” generally refers to electromagnetic radiation such as UV light, IR light, as well as visible light, which are commonly employed by optoelectronic sensors.
This state of the art is disadvantageous when the laser radar device monitors a large region where objects with widely differing reflection characteristics must be detected. Prior art laser radar devices therefore typically permanently employ very high signal dynamics for the light receiving arrangement and the associated signal processor. High signal dynamics entail high production costs and can functionally interfere with the laser radar device. Reasons for such functional interferences are, for example, external interfering light sources in the immediate vicinity of the monitored region that emit light which can reach the light receiving arrangement of the laser radar unit. Since the detection sensitivity of the light receiving arrangement must be set for the lowest expected signal strength generated by dark objects in the monitored region, such external light sources can significantly interfere with the proper functioning of the light receiving arrangement because the external light generates significant background noise which is superimposed onto the actual measurement signal. A similar effect occurs when the light pulse from the laser radar device strikes an object having a very high reflectivity and located in the immediate vicinity of the laser radar device. The resulting received signals generate saturation effects and/or background noise influences which significantly compromise the accuracy of the distance measurement. Prior art solutions for avoiding such problems with optoelectronic sensors seek to suppress the interference by conducting multiple measurements and, for example, reducing the measurement errors with average measurement values. It is also known to repeat incorrect measurements by changing the detection sensitivity of the light receiving arrangement. Such techniques cannot be used with laser radar devices that change the direction of the light because the beam direction of two or more successive light pulses are changed to such a degree that different objects or object segments are struck by them which cannot be related to each other. This can be reduced by correspondingly reducing the changes in the light directions, but this is not practically possible because the reaction time of the laser radar device would thereby be correspondingly reduced.